1. Field of the Invention
This invention relates generally to filtration cassettes, of a type comprising membrane filter sheets arranged in a multilaminate, peripherally bonded array wherein the filter sheets alternate with foraminous (e.g., screen or mesh) permeate sheet elements, and ribbed retentate channel elements. The invention further relates generally to cross-flow filters comprising a multiplicity of stacked filtration cassettes of such type, as well as to a method of making such filtration cassettes.
2. Description of the Related Art
Stacked plate cross-flow filters are utilized in a variety of solids-liquid separation operations, including the dewatering of solids-liquid suspensions such as aqueous biomass suspensions, the desalting of proteins, and the removal of secreted metabolites from cellular cultures.
In such systems, the stacked plates making up the cross-flow filter are typically mounted in a unitary frame structure whereby the respective plates are retained in alignment with one another, in a so-called "plate and frame" construction.
The plate and frame filter typically utilizes a unitary liquid feed conduit provided with openings at spaced intervals along its length and extending through the stacked plates as a means to introduce influent solids-containing liquid into the flow channels defined between adjacent plates in the stacked plate assembly. The flow channels in the plate and frame filter contain filter elements, such as disposable filter paper sheets or polymeric membranes, with which the solids-containing liquid is contacted and through which solids-depleted liquid passes. A unitary liquid withdrawal conduit featuring openings at spaced intervals along its length correspondingly extends through the stacked plates, in liquid flow communication with the respective flow channels of the stacked plate assembly, and conveys solids-depleted liquid out of the filter system.
As filtration proceeds, the filtered solids build up in the flow channels of the filter, on the "feed liquid sides", i.e., active filtration surfaces, of the filter sheets. The filter then is periodically backwashed, or alternatively, it may be fully shut down after a predetermined time or after a predetermined level of solids has accumulated in the flow channels on the filtration surfaces of the filter sheet elements, following which the system is drained of liquid, and the filter sheets replaced as necessary.
In one type of presently marketed stacked filter system, commercially available from Millipore Corporation (Bedford, Mass.) as the Prostak.RTM. cross-flow filter, the adjacent filter plates define a flow channel. Solids-containing influent liquid is fed at one side of the plate from a central location into a transversely extending feed distribution conduit, which is provided with openings at spaced-apart intervals along the length of the conduit for egress of the solids-containing liquid. At the opposite side of the adjacent plates, the flow channel is similarly constructed with a liquid collection conduit having openings along its length to collect the solids-depleted liquid and discharge same from a central outlet communicating with the collection conduit.
A major problem which has been encountered in cross-flow filters of the above-described type is that the liquid flow distribution, as for example reflected by the volumetric liquid flow rate or the liquid superficial velocity, is highly nonuniform in the transverse direction of the flow channel. Such maldistribution of the solids-containing liquid is a result of the fact that (1) the influent liquid is introduced into the feed distribution conduit at a central location, and (2) the cross sectional area of the liquid inlet is smaller than the sum of the inlet cross sectional areas of the cross flow subchannels.
Due to the pressure drop in the transverse direction, from the medial inlet port of filter plate assembly to the extremities of the feed distribution conduit, the local longitudinal flow (cross-flow) of liquid from the inlet side to the outlet side of the stacked plates, at progressively farther transverse distances from the central liquid inlet port, is progressively reduced in correspondence to the pressure drop experienced as the liquid is directed transversely to the outer extremities of the distribution conduit.
As a result, there is preferential channeling of the liquid at the central part of the flow channel from the inlet side to the outlet side thereof, and concomitant under-utilization of the peripheral areas of the filter. The available filter surface therefore is partially utilized in cross flow and partially in dead end filtration. When the solids in the central portion have been built up to a point requiring backwashing or cleaning of the filter, only that portion of the filter utilized in cross flow can be cleaned. The peripheral areas of the filter sheet remain fouled and cause carry-over of fouled material from one batch process to the next.
Such transverse maldistribution of the feed liquid in cross-flow filters of the aforementioned type could conceivably be overcome by the provision of header manifolds to introduce feed liquid into the filtration channels at multiple introduction points along the sides of the stacked filter plates, with a corresponding outlet header manifold arrangement at the opposite side of the stacked plates. Unfortunately, however, such provision would significantly increase the overall system pressure drop as well as the complexity of the filter system, since it would be necessary to positively seat the multiplicity of feed liquid branch lines passing from the manifold into the filter.
Another type of stacked plate cross-flow filter which has been commercialized employs a transversely extending liquid distribution conduit with spaced-apart openings therein to introduce solids-containing liquid into the flow channel between adjacent stacked plates, but instead of a central inlet port to flow the solids-containing liquid to such conduit, the liquid is axially fed into the conduit from a feed line connected to a transverse extremity of the conduit. Filters of such type are available from Millipore Corporation (Bedford, Mass.) under the trademark Pellicon.RTM.. This feed arrangement results in a progressive diminution of the liquid pressure at increasing transverse distances in the feed end of the distribution conduit, which in turn results in progressively transversely decreased cross-flow rates of liquid in the flow channel.
In an effort to overcome the aforementioned liquid flow maldistribution characteristics of stacked plate filters, filter plates have been constructed with baffle elements defining discrete flow channels, with the intent of achieving a more uniform distribution of the solids-containing influent liquid across the full areal extent of the filter elements in the flow channels of the filter.
A filter plate commercially available from Toyo Soda Manufacturing Company, Ltd. (Tokyo, Japan) has a structure in which solids-containing influent liquid is introduced to the flow channel at a central inlet port along side of the plate. A wall is disposed in front of the liquid inlet, extending upwardly from the floor of the flow channel and transversely toward the extremities of the flow channel, to divide the influent system into two outwardly directed streams. Downstream from such stream-splitting wall is a longitudinally extending divider partition, the steam-splitting wall and the divider partition together forming a "T" construction when viewed in plan view. Longitudinally spaced from a parallel to the stream-splitting wall are a series of baffle partitions on either side of the divider partition. The baffles extend transversely part-way across the flow channel on either side of the divider partition, so that there is formed a serpentine flow path for each of the split streams, on the respective sides of the partition. A unitary liquid outlet port is provided at the opposite side of the stacked plates from the inlet port, whereby the respective serpentine flows are finally joined and discharged from the flow channels of the filter.
Although the dual serpentine flow path arrangement described above provides a somewhat better distribution of liquid flow across the area extent of the filter paper element, the sharp turns in the flow path at the extremities of the baffles create edge and entrance effects in the flow streams which produce substantial dead space and bypassing therein. As a result of such anomalous flow phenomena, the filtration efficiency of the baffled serpentine flow arrangement is significantly reduced.
A microporous filter article is commercially available from Millipore Corporation (Bedford, Mass.) under the trademark MF-Millipore, as a stacked array comprising a top gasket sheet member, a filtrate separator plate, a filter sheet of microporous filtration medium, a retentate separator plate, a second filter sheet of microporous filtration medium, a filtrate separator plate, and a bottom gasket sheet member. The filtrate separator plates and the retentate separator plate include a peripheral border including manifold openings therein, circumscribingly enclosing a central screen or mesh flow channel area. In operation, a recirculating fluid path across the filter sheets is directed by the retentate screen, in the flow channel area of the retentate separator plate, which contacts the two filter sheets. The filter sheets separate the solution or other feed material to be separated, according to the porosity rating of the filter sheets. The filtrate screen, in the flow channel area of the filtrate separator plate, carries away the filtrate to the manifold openings in the peripheral borders of the filtrate separator plates. The filtrate then flows through the manifold openings of the filtrate separator plates to a filtrate collection means such as a vessel which is provided exteriorly of the filter comprising the stacked array of such filter articles.
Tangential ultrafiltration and microfiltration membrane cassette systems are available from Filtron Technology Corporation (Northborough, Mass.) under the trademarks Minisette, Centrasette, and Maxisette, which comprise multiple layers of membrane assemblies composed of ultra filtration or microfiltration membranes disposed between polymeric screen or linear open channel retentate separators and screen filtrate separators. Blocked borders on the filtrate and retentate screens direct the filtrate to collection ports on a bottom cell manifold. Cassettes can be provided in a stacked array, or cell, mounted between end plates, which may be provided with suitable ports, for introduction of feed material to be separated in the cassettes, and for discharge or withdrawal of filtrate and retentate.
My prior U.S. Pat. No. 4,882,050 issued Sep. 19, 1989 describes a filter plate characterized by substantially uniform transverse distribution of liquid from a unitary liquid feed port, and highly uniform liquid cross-flow across the full transverse extent of the flow channel formed when plates of such type are stacked to form a cross-flow filter.
The filter plate of this prior patent has a generally planar and rectangular shape with a substantially flat bottom surface. A top surface of the plate is provided with an upwardly extending wall circumscribingly bounding a flow channel of generally rectangular shape. A liquid inlet port is disposed at a medial part of a first side of the flow channel, with the liquid outlet port at a medial part of a second side of the flow channel opposite the first side thereof. The liquid inlet port is joined in liquid flow communication with a liquid feed trough extending transversely across the first side of the flow channel, and the liquid outlet port is joined in liquid flow communication with a liquid collection trough extending transversely across the second side of the flow channel.
In this construction, a plurality of spaced-apart partitions extend upwardly from the floor of the flow channel between the liquid feed trough and the liquid collection trough. These partitions are of lesser height than the walls circumscribing the flow channel and are substantially parallel to one another, to define a series of sub-channels extending longitudinally between the liquid feed trough and the liquid collection trough. Both the liquid feed trough and the liquid collection trough are of progressively increasing depth from their respective medial portions to their marginal extremities.
Plates according to this prior patent may be utilized in stacked pairs to form enclosed flow channels within which filtration may take place in a highly efficient manner. Specifically, a first plate of the type broadly described above is paired with a structurally identical second plate positioned in inverted relationship to the first plate, such that the respective circumscribingly bounding walls of the first and second plates are in abutting sealing contact with one another. In this stacked arrangement, a filter element support of generally rectangular shape approximating the dimensions of the flow channel is interposed between the adjacent plates, with filter sheet elements between the support and each of the respective pair filter plates.
My prior U.S. Pat. No. 4,867,876 discloses a filter plate suitable for use with filter elements to form a stacked plate filter. In the stacked plate filter, pairs of such filter plates are mated with filter elements therebetween, to form flow channels wherein solids-containing liquid may be contacted with the filter sheet elements for filtration thereof to produce solids-reduced liquid, and permeate.
The filter plate of U.S. Pat. No. 4,867,876 has a generally planar shape with a substantially flat bottom surface. A top surface of the plate is provided with a first upwardly extending wall circumscribingly bounding a flow channel of generally rectangular shape. The flow channel in this prior design is circumscribingly bounded by a second upwardly extending wall interior to and of lesser height than the first circumscribingly bounding wall, the second wall being in spaced-relation to the outer wall along diagonally opposed L-shaped peripheral sections of the flow channel, each such L-shaped peripheral sections comprising a leg extending transversely across the flow channel for a major portion of the length thereof, and a leg dimension of the flow channel and communicating at its extremity with an opening extending through the plate, with the portions of the periphery of the flow channel not comprising such L-shaped sections comprising ridge elements extending between the first and second circumscribingly bounding walls.
A liquid inlet port is disposed at a first side of the flow channel in this prior design, with a liquid outlet port at a second side of the flow channel opposite the first side thereof.
The liquid inlet port in this prior filter plate is joined in liquid flow communication with a liquid feed trough interior to the second bounding wall and extending transversely across the first side of the flow channel. The liquid outlet port is joined in liquid flow communication with a liquid collection trough interior to the second bounding wall and extending transversely across the second side of the flow channel.
A plurality of spaced-apart partitions extend upwardly from the floor of the flow channel between the liquid feed trough and the liquid collection trough. Such partitions are substantially parallel to one another to define a series of subchannels extending longitudinally between the liquid feed trough and the liquid collection trough. These partitions preferably are of lesser height than the first (outer) wall circumscribing the flow channel and of substantially the same height as the second (inner) wall circumscribing the flow channel.
Plates of the foregoing type may be utilized in stacked pairs to form enclosed flow channels within which filtration may take place in a highly efficient manner. Specifically, a first plate of this type broadly described above may be paired with a plate of the type broadly described above may be paired with a corresponding second plate positioned in inverted relationship to the first plate, such that the respective first circumscribingly bounding walls of the first and second plates are abutting sealing contact with one another. In such stacked arrangement, a filter element of generally rectangular shape approximating the dimensions of the flow channel is interposed between the adjacent plates, suitably with its peripheral edges reposed on the second bounding wall. Such filter element is provided with an interior flow structure, whereby permeate entering the interior of the element is conveyed to the edge portions of the element for discharge into the aforementioned L-shaped peripheral sections of the flow channel between the respective first and second bounding walls. In an illustrative aspect, the filter element may comprise a foraminous support of generally rectangular shape approximating the dimensions of the flow channel, interposed between the adjacent plates, with filter sheet elements between the foraminous support and each of the respective filter plates.
In the operation of a stacked filter plate assembly of the type disclosed in prior U.S. Pat. No. 4,867,876, liquid introduced via the liquid inlet port enters the liquid feed trough and is laterally distributed from the associated portion of the feed trough to outer extremities thereof. The liquid flow is directed into the sub-channels to yield a longitudinal liquid cross-flow which is highly uniform over the full transverse extent of the flow channel, so that the full areal extent of the filter element is highly effectively utilized. As a result, the solids filtration capacity of the stacked plate assembly is substantially increased and the assembly is capable of significantly extended operation prior to regeneration of the filter, as compared to various prior art cross-flow plate and frame filters.
My prior U.S. Pat. No. 5,049,268 discloses a filter plate which may be used in invertedly facing pairs, with a sheetholder having filter sheets disposed on its respective faces, mounted between the facing filter plates. The filter plate is characterized by (a) a generally rectangular and generally planar shape with main top and bottom surfaces; (b) a first wall extending upwardly from said main top surface and circumscribingly bounding a flow channel area of generally rectangular shape thereon; (c) a second wall on said main top surface extending upwardly therefrom, said second wall being interior to and of lesser height than said first wall and in spaced relation to the first along diagonally opposed L-shaped peripheral sections of the flow channel area; (d) each L-shaped peripheral section comprising a leg extending transversely across the flow channel area for a major portion of the width thereof, and a leg extending longitudinally for a portion of the length of the flow channel area and communicating at its extremity with an opening extending through the plate, with peripheral portions of the flow channel area not comprising such L-shaped sections comprising ridges extending between the first and second walls; (e) an inlet port opening at a corner portion of the filter plate at a first end thereof, extending through the plate, interiorly positioned in a polygonal-shaped distribution basin on the main bottom surface of the plate, said distribution basin being bounded by generally linear side edges defining corners of the basin at respective intersections of the side edges; (f) an inlet trough opening extending through said plate and transversely across a major portion of the width of said flow channel area and along a first side edge of said distribution basin, such that said liquid inlet trough opening communicates said distribution basin on the main bottom surface of the plate with said flow channel area on the main top surface of the plate; (g) an outlet port opening at a corner portion of said plate diagonally opposite said inlet port opening, said outlet port opening extending through the plate, interiorly positioned in a polygonal-shaped collection basin on the main bottom surface of the plate, said collection basin being bounded by generally linear side edges defining corners of the collection basin at respective intersections of the side edges thereof; (h) an outlet trough opening extending through the plate and transversely across a major portion of the width of said flow channel area and long a first side edge of said collection basin, such that said outlet trough opening communicates said collection basin on the main bottom surface of the plate with said flow channel area on the main top surface of the plate; and (i) a plurality of transversely spaced-apart partitions extending upwardly from the floor of the flow channel area between the liquid feed trough and the liquid collection trough, said partitions being of substantially the same height as the second wall and substantially parallel to one another to define a series of channels between the partitions, extending longitudinally between the liquid feed trough and the liquid collection trough.
In one specific embodiment disclosed in prior U.S. Pat. No. 5,049,268, the liquid distribution basin and liquid collection basin each have quadrilateral shape, wherein each basin comprises: the port opening being disposed at a first said corner and the side edges intersecting at the first corner defining a first included angle w therebetween of from about 60.degree. to about 110.degree.; a second corner diagonally opposite the first corner, and the side edges intersecting at the second corner defining a second included angle x therebetween of from about 45.degree. to about 90.degree.; a third corner transversely adjacent the first corner and longitudinally adjacent the second corner, with the side edges intersecting at the third corner defining a third included angle y therebetween of from about 70.degree. to about 135.degree.; a fourth corner longitudinally adjacent the first corner and transversely adjacent the third corner, with the sides edges intersecting at the fourth corner defining a fourth angle z therebetween of from about 60.degree. to about 90.degree.; and the side edge extending transversely between the second and fourth corners also bounding the associated trough opening extending through the plate and communicating the basin with the flow channel area.
The filter plate disclosed in prior U.S. Pat. No. 5,049,268 is very highly efficient in effecting mass transfer operations, e.g., dewatering of aqueous biomass suspensions, desalting of proteins, and removal of secreted metabolites from cellular suspensions. Such filter plate may be readily formed as a unitary article by injection molding thereof from a variety of plastics materials, such as polypropylene, polysulfone, polyvinylchloride, etc., but due to the relatively complex geometric character of the filter plate, the injection molded plates are relatively expensive to produce.
Accordingly, it would be a significant advance in the art to provide a filtration article of a type which provides the superior mass transfer efficiency and utility of the filter plate disclosed in prior U.S. Pat. No. 5,049,268, but which is much more economical to produce than the filter plate and sheetholder of such prior U.S. Pat. No. 5,049,268.
It therefore is an object of the present invention to provide such an improved filtration article.
It is another object of the invention to provide a filter comprising a stack of improved filtration articles of such type.
It is another object of the invention to provide a method of making such improved filtration article, in a simple, efficient, and economical manner.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.